1. Field of the Invention
The invention concerns a galvanizing tank with an outer tub of steel, within which an inner tub of chemically resistant plastic is seated.
For chemical or electrochemical surface treatment of parts, the parts with their surfaces to be treated are dipped into baths comprising aggressive media, for example acids or lye.
Because of the static loads, it is preferred that the bath be provided in a tub of a sturdy material, for example of steel or concrete. However, materials with a sufficiently high sturdiness have the disadvantage that they are attacked by the aggressive media and are destroyed, while materials resistant to aggressive media do not exhibit sufficient stability. In order to combine the static load bearing characteristics with a reliable protection against aggressive media, two solutions are known.
The first approach to solving the problem envisions that the tub of the static load bearing material, for example a steel tub, is cladded with a chemically resistant plastic, while in the second approach an inner tub of a chemically resistant plastic is seated in this steel tub. The present invention is concerned with this second approach.
The aggressive media is located in an inner tub, frequently referred to as inner liner, of chemically resistant material, for example of plastic, which is seated in an outer tub made for example of steel or concrete. Depending on the type of the surface treatment, media in the inner tub is heated to a temperature of 100.degree. C. An arrangement comprised of an outer tub and an inner tub seated in the outer tub for receiving an aggressive media is thus subject to multi-faceted requirements and strong stresses. The inner tub must on the one hand be resistant against the aggressive media. Because of the different thermal coefficients of expansion of the inner and the outer tub, which are made of the different materials--plastic for the inner tub, steel for the outer tub--there must on the other hand be provided suitable precautionary means, in order to prevent mechanical damage, for example tears, of the two tubs as well as a leakage or overflow of the aggressive media.
2. Description of the Related Art
From DD 213 698 B1 an arrangement of this type, with an outer tub and an inner tub, is known.
In the outer tub with a rim made of steel is seated an inner tub of polypropylene with narrower breadth and width than the internal breadth and length of the outer tub, in order to provide sufficient space for lengthwise and widthwise expansion of the inner wall caused by rising temperature. On the upper edge of the inner tub a trapezoid-shaped circumscribing shank is welded on, which lies on the rim of the outer tub. The inner kink-bend of the shank is directed upwardly and curved outwards, so that the inner wall can expand upwardly, the shank however nevertheless remains resting upon the rim of the outer wall.
A first disadvantage of this known arrangement lies therein, that the trapezoid-shaped shank lying on the rim of the outer tub has no flat or planar resting surface, but rather exhibits an arc directed upwardly. A second disadvantage is to be seen therein, that this arc changes with temperature changes.
In DD 145 641 a further arrangement of an outer tub and an inner tub is known.
The inner wall of plastic exhibits raised weld beads, on which the inner tub rests upon the outer tub, in order to provide space for the expansion of the inner wall as a result of increases in temperature.
In this arrangement there is the danger, that fluids running out of the inner wall can flow into the space between the inner and outer tub.
Finally, from DD 234 448 A1 a further arrangement of an outer and an inner tub is described.
One plastic tub seated in a steel tub lies only in the upper rim area and only partially on the floor on the steel tub, so that the plastic tub can expand in a pillow-like manner as a consequence of increases in temperatures.
Also in this arrangement there is the danger that overflowing fluids out of the inner tub flow into the space between the inner and outer tub.